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Abstract
In this work, copper circuits were fabricated on flexible polyimide (PI) substrates by high repetition rate femtosecond laser-induced selective local reduction of copper oxide nanoparticles (CuO NPs). The effects of laser pulse energy and laser scanning velocity on the quality of the copper circuit were studied. By optimizing laser processing parameters, we prepared a Cu circuit of a line width of 5.5 µm and an electrical resistivity of 130.9 µΩ·cm. The Cu/O atomic ratio of the Cu circuit reaches ∼10.6 and the proportion of Cu is 91.42%. We then studied the formation mechanism of the copper circuit by simulating the temperature field under the irradiation of high repetition rate femtosecond laser pulses. The results show that the thermochemical reduction reaction induced by the high repetition rate femtosecond laser reduces CuO NPs into Cu NPs. Under the thermal effect of the high repetition rate femtosecond laser, Cu NPs agglomerate and grow to form a uniform and continuous Cu circuit.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to the rapid development of flexible electronics, a variety of fabrication methods have been proposed to fabricate copper circuits on flexible polymer substrate [1–3]. As a nonvacuum and maskless fabrication method, laser direct writing of conductive circuits has drawn much attention in recent years. Several laser direct writing techniques have been proposed to fabricate metallic circuits on polymer surfaces, including laser direct structuring (LDS) [4–6], laser-induced selective activation [7,8] and laser-induced selective sintering of metal nanoparticles ink (NPS) [9–11].
Among these methods, LDS is a two-steps process. There are a few commercial materials for LDS available on the market, but most of them are based on expensive metal-organic filler. Xu [11] reported space-selective metallization of 3D microfluidic structures using femtosecond laser direct-writing ablation followed by electroless metal plating. The palladium colloidal activator, which is commonly used in laser-induced selective activation, is a kind of material with ultra-high surface activity. Ratautas [7,8] combined laser activation and electroless plating to prepare a Cu circuit on the polypropylene. The narrowest width of a copper-plated line was less than 23µm. Different from those two methods mentioned above, laser-induced metal precursor NPs ink sintering combines “local treatment” with “circuit manufacturing” to complete the preparation of conductive patterns in one step. Khalilur Rahman [12] studied the influence of green laser parameters on the sintering of commercial CuO NP inks and prepared Cu lines with resistivity ranged from 9.5 to 20 µΩ·cm. Hernandez-Castaneda [13] studied the feasibility of sintering non-conductive copper nanoparticles on polyethylene terephthalate polymer films with different laser systems. Mizoshiri [10] achieved direct writing of Cu-rich micro temperature sensors on PDMS, but their resistivity was as high as 1.9×10−4 Ω·m, which was 4 orders of magnitude larger than pure Cu and the minimum line width was 15 µm. Koritsoglou [14] successfully produced an on-chip antenna on a flexible integrated circuit through the combination of laser printing and laser sintering of copper nanoparticle ink, with a resistivity as low as 3.3 µΩ·cm. Seunghyun [15] showed that a laser-induced hybrid process of reductive sintering and adhesive transfer can be used to cost-effectively fabricate copper electrodes on a polyethylene film with a thermal resistance below 100 °C. The mechanism of laser sintering of Cu precursor inks in air has been reported by Ohishi and Yu [16,17]: the high energy and heat supplied by the laser can decompose the Cu precursors rapidly, and since the Cu particles formed in situ are minute, they melt and grow into larger stable Cu particles before they are oxidized. Laser direct writing techniques can fabricate metallic circuits on polymer surfaces. However, the cost of precious metals (Au, Ag, and Pt) is relatively high, and metal circuits with high resolution are prepared by these methods. Cu is a perfect substitute. However, Cu NPs are easier to be oxidized in air, so the inert or vacuum environment and additional seal are normally needed to prevent the oxidization during the process of synthesis, storage and fabrication of Cu NPs, which increases the complexity of the device and the processing cost of materials [18–21]. In a word, it is still a great challenge to manufacture flexible circuits with high resolution and low cost by laser direct writing techniques [22].
In this study, we propose a new method to prepare copper circuits on flexible polymer substrates by high repetition rate femtosecond laser-induced selective local reduction of CuO NPs. This method uses air-stable CuO NPs with high cost effectiveness. In addition, water-soluble ethylene glycol was used as a reducing agent to prepare a dispersed CuO NPs ink. This method uses high repetition rate femtosecond laser as the light source. Since the flexible copper circuits fabrication is processed under multiple pulses irradiation at a high repetition rate (76 MHz), the heat accumulation can be controlled to near the selective local reduction of CuO NPs threshold [23]. In addition, the effects of different laser processing parameters on the quality of Cu circuit are studied, and the applications of this method on flexible polymer substrates is verified. The mechanism of femtosecond laser-induced reduction and sintering is discussed via finite element numerical simulation.
2. Experimental
2.1 Setup
Figure 1(a) depicts the femtosecond laser fabrication of flexible circuit system. The femtosecond laser (PHAROS from Light Conversion) with a wavelength of 1030 nm, a pulse duration of 100 fs, a repetition rate of 76 MHz, and a maximum output power of 1W is used as the laser source. The output laser beam passes through a mechanical shutter, a half-wave plate, a polarized beam splitter and is then focused by a 10X infinity corrected objective with N.A of 0.42 (Mitutoyo). Placing the sample on an X-Y moving stage (Aerotech ANT130), and we realize various motion trajectories by controlling the computer. A coaxial CCD is used to observe the processing situation online.
Fig. 1. (a) Schematic diagram of femtosecond laser fabrication of flexible circuit system; (b) The experiment flow chart for the flexible Cu circuit array prepared by femtosecond laser-induced selective local reduction of CuO NPs ink.
2.2 Processing
The experimental procedures are summarized in Fig. 1(b). The CuO NPs (from Alfa Aesar Co.) as a metal precursor material has an average size of 25 nm. The CuO NPs (50 wt %) was dispersed in a solution consisting of polyvinylpyrrolidone (PVP average molecular weight 24000, 16.25 wt %) and ethylene glycol (EG, average molecular weight 62.068, 33.75 wt %) with an ultrasonic wave. The PVP acts as a surfactant and the EG is used as a reduction agent. With the combined action of water bath heating magnetic stirring and ultrasonic vibration, a kind of transparent dispersant solution with certain adhesion consisting of PVP and EG is prepared. Then, by mechanical vibration, magnetic stirring, and ultrasonic vibration, CuO NPs is dispersed in the solution at a certain ratio to prepare the black CuO NPs ink, with a thickness of about 7 µm. The plasma-treated PI film is fixed on a glass substrate, and then the CuO NPs ink is spin-coated on the PI film. The as-prepared samples are then put in an oven at 50 °C for 30 minutes to remove the unnecessary moisture to prepare the CuO NPs coating. Femtosecond laser is used to fabricate micro-patterns on the CuO NPs coating, and the samples are rinsed with ethanol and deionized water to remove the untreated CuO NPs, therefore, a flexible Cu circuit array is obtained on the surface of the PI film.
2.3 Characterization
The wettability of the PI substrate is evaluated by measuring the contact angle of droplets on the substrate with a video-based optical contact angle-measuring device (OSA200S-T from NBSI). The 3D profile of the prepared pattern is measured by using a laser-scanning confocal microscope (OLS4000 series from Olympus). The shape and size of Cu circuit are measured with an optical microscope and a scanning electron microscope (SEM, SU8010 from Hitachi). In addition, an energy-dispersive spectrometry (EDS, SU8010 from Hitachi) measurement is conducted to quantitatively investigate the major chemical composition (C, O, Cu) ratio in the Cu circuit. Field emission transmission electron microscope (TEM, Talos F200S from FEI and Thermo) is used to analyze the micro-morphology, crystal structure and phase structure of Cu NPs at atomic-scale resolution.
3. Results and discussion
Figure 2(a) shows the X-ray diffraction (XRD) analysis results of the chemical composition of CuO NPs. It can be seen from the figure that it is pure CuO NPs without doping. The CuO NPs agglomerated seriously before the dispersing agent was added [Fig. 2(a) inset], while CuO NPs were mutually independent after adding [Fig. 2(b)]. The size of the CuO NPs in the colloidal solution was measured to be approximately 30-50 nm, as shown in Fig. 2(b). The PI substrates are treated by oxygen plasma to improve the surface wettability. The contact angle measurement system was used to measure the wettability of the PI film surface with glycerol and distilled water by the fixed drop method, as shown in Fig. 2(c), which represents the contact angle of the PI film surface after 1 min of oxygen plasma treatment.
Fig. 2. (a) XRD analyses of chemical composition of CuO NPS, (Inset) SEM image of CuO NPs; (b) TEM characterization of CuO NPs dispersion with the addition of PVP; (c) The surface contact angles of PI film after oxygen plasma treatment, propanetriol: 32.3°; distilled water: 35.7°.
3.1 Optimization of processing parameters and characterization of patter
Figure 3 shows the surface morphology and content of the Cu circuit under different laser pulse energies when the scanning velocity is 15 mm/s and the CuO NPS coating thickness is about 7 µm. When the laser pulse energy is 0.1 nJ, the Cu circuit possesses the poor connection [Fig. 3(a)]. High-magnification SEM pictures [Figs. 3(a1) and 3(a2)] show that there are a number of large holes and gaps between Cu NPS. With the increase of laser pulse energy, especially when it is as large as 0.17 nJ and 0.24 nJ, the contact between Cu NPS is inclined to tight [Fig. 3(b) and 3(c)] and the high-magnification SEM pictures [Figs. 3(b1), 3(b2), 3(c1). and 3(c2)] indicate that the adjacent Cu NPS are well connected.
Fig. 3. (a) ∼ (c) are the metallographic diagrams of Cu circuit at different laser pulse energy; (a1) ∼ (c1) SEM images; (a2) ∼ (c2) High resolution SEM images of Cu circuit.
EDS element content of Cu circuit [Fig. 4(a)] indicates that when the laser pulse energy is increased from 0.1 to 0.17 nJ, although the Cu line is more uniform and much denser, the content of Cu/O almost remained unchanged. It is because the laser pulse energy is insufficient to completely reduce CuO NPs to Cu NPS, so some Cu2O remained in the reduced Cu line. When the laser pulse energy increases to 0.24 nJ, the atom of Cu/O is increased from 5.5 to 13.5, and the relative content of Cu/O is significantly increased. Therefore, the surface connectivity of Cu circuit is even better in Fig. 3(c) than that shown in Figs. 3(a) and 3(b). Under effective experiment parameters, the line width of the Cu circuit increases approximately linearly with laser pulse energy [Fig. 4(b)]. High laser energy causes a violent laser-induced photothermal chemical reduction reaction, which makes the heat-affected zone in the NPS sintering process larger and causes the circuit line width to increase.
Fig. 4. (a) Influence of Cu circuit element composition by single pulse energy; (b) Influence rule of Cu circuit line width by single pulse energy.
In summary, when the laser single pulse energy is 0.24 nJ, the Cu circuit has the best surface morphology and the Cu atom proportion is the highest and the electric conductivity is the highest. However, the corresponding line width of Cu circuit is relatively large. Therefore, it is necessary to optimize the laser processing parameters by combing the scanning velocity so as to fabricate the Cu circuit with high conductivity, high quality, and high resolution.
Figure 5 shows the laser reduction and sintering of the Cu circuits under different scanning velocities when the laser pulse energy is 0.24 nJ, the thickness of the CuO NPS coating is about 7 µm, and the fixed pulse repetition rate is 76 MHz. When the scanning velocity varied within a range of 1∼20 mm/s, the overlapping rate of focus spot is not less than 99.99%.
Fig. 5. (a)–(c) The metallographic diagrams of Cu circuit under different scanning velocities; (a1)–(c1) SEM images; (a2)–c2) High resolution SEM images of Cu circuit; (a3)–(c3) Characterization of the porosity of Cu circuits.
When the scanning velocity is 3 mm/s, as CuO NPs has a stronger absorption to on near infrared light than Cu NPS, the obvious ablation defects can be observed in the laser scanning trajectory under the low scanning velocity [Figs. 5(a1) and 5(a2)]. The high-magnification SEM picture [Fig. 5(a3)] shows that the Cu NPS are completely melted and sintered to agglomerates of nano-particles due to laser thermal effect when the scanning velocity is too low.
When the scanning velocity increases to 7 mm/s, it can be observed that agglomerates of nano-particles and nano-particles are coexisting in the Cu circuit [Figs. 5(b3) and 5(c3)], this point, the Cu circuit has the relatively better morphology. With the increase of scanning velocity, the porosity of Cu circuit decreases from 25.30% to 21.91%, and then to 15.52% [Figs. 5(a3), 5(b3) and 5(c3)], which matches well with the Cu circuit optical microscope images in Figs. 5(a)–5(c).
The effect of scanning velocity on the EDS element composition of Cu circuit is shown in Fig. 6(a). When the scanning velocity is low as 3 mm/s, one part of CuO NPs are reduced to Cu NPs, another part is removed by laser ablation, contributing to the consequence that the organic matters in the CuO NPS coating remain in the Cu circuit because of the lack of metal precursor material. Therefore, the C content is almost equivalent to Cu content in the Cu circuit, both of which account for nearly 50%. With the increase of the scanning velocity, the proportion of Cu significantly increases and the C rapidly decreases, which indicates that most of CuO NPs has been deoxygenized to Cu NPs in the reaction process with organic composition.
The effect of laser scanning velocity on the line width of Cu circuit is shown in Fig. 6(b). The line width is reduced to the scanning velocity and is inversely proportional to it. The laser energy received per unit irradiation area reduces with the increase of scanning velocity, making for weaker photothermal chemical reduction reaction and thermal diffusion, which ultimately leads to a smaller line width.
On the basis of the above experiments, it is determined that the optimized patterning condition is a laser pulse energy of 0.17 nJ, scanning velocity of 5 mm/s, CuO NPs coatings with a thickness of about 7 µm. Figure 7 shows the images of various copper electrodes created by the optimized conditions on a flexible polymers substrates. The line width of Cu circuit was 11 µm and the Cu line height was consistent with CuO coating thickness of 7 µm [Fig. 7(a)]. The Cu circuit resolution can reach about 5.5 µm by either reducing the laser single pulse energy or increasing the scanning velocity. However, in this case, its electricity resistance becomes considerable. The electrical resistance of Cu circuit with a length of 2 mm is measured to be 37.4 Ω, from which the electrical resistivity ρ of 130.9 µΩ·cm is calculated (Ignore the uneven surface of Cu circuit). Although its resistivity is 2 orders of magnitude higher than that of pure Cu, it can still meet the practical applications of high-quality electronic equipment. Secondary products, including diacetyl, and water are generated during the process of laser reduction of CuO NPs, and its volatilization leads to the formation of pores in the Cu circuit, resulting in the resistivity of the Cu circuit higher than that of pure Cu. According to the research of Md. Khalilur Rahman et al., we can increase the resistivity of the laser induction circuit by changing the thickness of the CuO coating and the baking times [12]. Based on the preparation of Cu circuit arrays, this method has successfully prepared microelectrodes and Zig-zag-type conductive patterns [Figs. 7(c) and 7(d)]. According to the rinsing conductive pattern [Figs. 7(c1) and 7(d1)] and the partially enlarged conductive pattern, the surface morphology of the complex conductive pattern before and after rinsing is continuous and complete. The adhesion between complex conductive patterns and flexible polymer substrates is enough for industrial applications. It is confirmed that the proposed process can form conductive patterns with various shapes and lengths.
Fig. 7. Photograph of Cu circuits on the flexible polymer substrate. (a) Cu circuit array. (Inset) Cu circuit after rinsing; (b) SEM image; (b1) Porosity characterization of Cu circuit; (c) Cu microelectrodes; (d) Spider-type conductive pattern. (Inset) microscopic image; (c1-d1) Respectively (c-d) corresponds to the circuit pattern after rinsing; (e) EDS diagram. (Inset) Cu circuit element distribution.
After being optimized with the laser processing parameters, the Cu/O atomic ratio of the Cu circuit reaches ≈10.6 and the proportion of Cu is 91.42% [Fig. 7(e)]. Except for the content of Cu and O, there are no other C impurity elements in the Cu circuit, which indicates that almost all of CuO NPS have been fully deoxygenized and sintered into Cu NPS, and the secondary product generated in the reaction has also been removed. In the experiment, the Cu circuit with high purity and high density of which the porosity is only 9.89% [Fig. 7(b1)] has been obtained, as shown in Fig. 7(b).
Figure 8(a) shows a high-resolution TEM image used to characterize Cu NPS. As can be seen from the figure, the lattice fringe spacing of Cu NPS is 0.21 nm, corresponding to the lattice plane of the face-centered cubic Cu. The selected area electron diffraction (SAED) of Cu NPS is shown in Fig. 8(b), in which the aureole can be seen clearly. The lattice fringe spacing can be calculated by measuring the reciprocal value of the diameter of aureole. By comparing the Jade standard card, it can be found that the lattice fringe spacing is consistent with the face-centered cubic of Cu, which corresponds to the Cu lattice plane of (111), (200), (220), and (311) of Cu from inside to outside, respectively. In summary, the thermochemical reduction reaction induced by the high repetition rate femtosecond laser-induced is basically consistent with the numerical simulation of the temperature field of the CuO NPs ink reduction and sintering process.
Fig. 8. Cu circuit element distribution: (a) High resolution TEM image of Cu NPS; (b) SAED image of Cu NPS.
3.2 Mechanism of femtosecond laser-induced reduction and sintering
For the materials to absorb the laser energy via interband absorption, lasers with photon energies equal to or higher than the bandgap of the material are required. Because the bandgap of CuO is 1.2 eV, a near-infrared (NIR) femtosecond laser with a wavelength of 1030 nm ($1.204\; eV$) can be utilized to process CuO NPs. Figure 9(b) shows the absorption spectrum of CuO NPS coating from visible light to near-infrared wavelength. The absorbance of the CuO NPs coating is 0.665 at the wavelength of 1030 nm. When laser pulse energy is 0.17 nJ, scanning velocity is 5 mm/s, scanning times is 1 and CuO NPS coatings with a thickness is about 7 µm, the finite element model is used to analyze the temperature field of the femtosecond laser-induced CuO NPs coating reduction and sintering process by numerical simulation (Supplement 1), as shown in [Figs. 9(a) and 9(c)]. It can be seen from Fig. 9(a) that the CuO NPs coating can reach a maximum temperature of 262.2°C under the action of a femtosecond laser, and its temperature remains basically unchanged during the processing.
Fig. 9. (a) Typical temperature field distribution of CuO NPS coating after femtosecond laser irradiation at 0.62s; (b) Absorption spectrum of the CuO NPS coating; (c) Changes in temperature over time during processing; (d) Schematic diagram of mechanism of femtosecond laser-induced reduction and sintering of CuO NPS ink.
Based on the above analyses, laser irradiation on CuO NPs can induce thermochemical reduction reaction to prepare Cu circuits. A schematic diagram depicting the reaction mechanism of the femtosecond laser induced reduction and sintering of CuO NPs ink for the electrical Cu circuits are shown in Fig. 9(d).
- (1) CuO NPs absorb laser energy and convert photon energy into thermal energy. CuO NPs coating changes from gray-black to yellow. When the temperature of ink reaches 160-200°C (close to the boiling point of glycol), the organic solvent, EG starts to dehydrate and generate the acetaldehyde, which can be used to reduce CuO NPs to Cu NPS. The chemical equation is shown as below:
- (2) Due to the thermal effect of the high repetition rate femtosecond laser, the high-intensity energy transferred during the laser sintering heats the Cu NPS to 260°C [Fig. 9(a) basically consistent with numerical simulation temperature]. At this temperature, the Cu NPs surface atoms begin to move, and under the action of van der Waals force, a neck-like structure is formed between the Cu particles.
- (3) Particles are agglomerate and grow to form the Cu layer, which provides an even and continuous route for the electron flow. As a result, the uniform and continuous Cu circuit is generated.
4. Conclusions
In this paper, CuO NPs ink with excellent dispersity and moderate adhesion is prepared. Combined with experimentals and numerical simulation, the mechanism and application of femtosecond laser-induced selective local reduction are studied. The impacts of laser pulse energy and scanning velocity on the quality of Cu circuits are discussed, and a technique of Cu circuits fabricated on flexible polymer substrates by high repetition rate femtosecond laser-induced selective local reduction of CuO NPs has been obtained. Optimized laser processing parameters (laser pulse energy of 0.17 nJ, scanning velocity of 5 mm/s, scanning times of 1, CuO NPs coatings with a thickness of about 7 µm) are used to realize a Cu circuit with high purity, high conductivity, high density, and high resolution on flexible PI substrates. The measured Cu content in the Cu circuit is 91.416%, the porosity 9.89%, the electrical resistivity 130.9 µΩ·cm, and the line width can be as small as 5.5 µm, which is good enough to meet the practical needs of high-quality electronic devices.
Funding
National Natural Science Foundation of China (51805093, 52075103); Key Project of Regional Joint Fund of Guangdong Basic and Applied Basic Research Foundation (2020B1515120058); National Key Research and Development Program of China (2018YFB1107700).
Acknowledgments
This research was performed at Laser Micro/Nano Processing Lab, School of Electromechanical Engineering, Guangdong University of Technology.
Disclosures
The authors declared that there is no conflict of interest.
Supplemental document
See Supplement 1 for supporting content.
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